The cable television network operators industry established and promulgated the data over cable system interface specification (DOCSIS) standard or protocol, including the DOCSIS 3.0 standard, to facilitate transporting data traffic, primarily Internet traffic, over existing community antenna television (CATV) networks. In addition to transporting data traffic, as well as television content signals over a CATV network, multiple services operators (MSO) also use their CATV network infrastructure for carrying voice, video on demand (VoD) and video conferencing traffic signals, among other types.
In general, DOCSIS specifies interface requirements for equipment involved in data distribution over cable television system networks. In a cable telecommunications system, video and data content can be digitized and then compressed according to one or more standards, for example MPEG, MPEG2, etc. The infrastructure for processing and delivering the content typically can include an access node, which may be, for example, a modular cable modem termination system (M-CMTS) that communicates with one or more user devices. Typically within the M-CMTS, the M-CMTS core communicates with an edge quadrature amplitude modulation (EQAM) device, which can receive incoming transport streams, process the content into an MPEG transport stream, and modulate the stream onto an RF carrier for delivery through an access network to a network element user device, which can be located in a subscriber's home.
The M-CMTS core can communicate with the EQAM via the DOCSIS data downstream external physical interface (DEPI), which is an IP tunnel interface that contains not only a data path for DOCSIS frames, but also a control path for the set-up, maintenance, and tearing down of sessions. DEPI uses as its base protocol the layer 2 tunneling protocol version 3, or L2TPv3 for short. L2TPv3 is an Internet Engineering Task Force (IETF) generic protocol for creating a “pseudowire,” which serves as a mechanism that can transparently transport a layer 2 protocol over a layer 3 network.
The M-CMTS core converts packets it receives into DOCSIS compatible packets, which can then be encapsulated into DOCSIS MPEG transport (DOCSIS MPT) or DOCSIS packet streaming protocol (DOCSIS PSP) packets. In the case of MPT mode, multiple 188-byte MPEG2 transport stream (MPEG-TS) packets are transported in the L2TPv3 tunnel payload. In MPT mode, IP content is encapsulated into DOCSIS MAC frames and the DOCSIS MAC frames are encapsulated into MPEG-TS packets by the M-CMTS core. On the other hand, in PSP mode, raw DOCSIS frames are sent to the EQAM. The encapsulated packets are formatted according to the L2TPV3 protocol, and received by the EQAM.
The EQAM can receive the packets and extract the DOCSIS compatible packets (which may be video or data) within the DEPI payload and re-packetize the digital content into an MPEG-TS. It then uses QAM modulation to digitally modulate the digital transport stream onto a downstream RF carrier, which is sent via downstream RF interface (DRFI) to a user device.
For the processing of MPEG-TS packets, such as with the DOCSIS MPT mode, an EQAM device typically uses de-jitter buffer memory to buffer the transport stream so that any delay or congestion that was introduced in the transport stream is removed.
DEPI L2TPv3 PSP sessions are typically mapped 1:1 per QAM channel, and each session supports up to 8 separate priority queues, typically serviced on a strict priority de-queuing basis. When multiple priorities are used within a PSP session, the receiving device (normally an EQAM) will have to assign prioritized packet data to the configured priority queues, which are emptied (and processed for downstream transmission) on a strict priority de-queuing basis.
The EQAM can additionally be used to deliver video streams in MPEG-TS format to user devices, such as set top boxes. In this use, the EQAM can receive non-DOCSIS MPEG elementary streams that have been encapsulated in MPEG packets and placed in a user datagram protocol (UDP) message, a path that can by-pass the M-CMTS core processing.
Thus, the processing of the non-DOCSIS MPEG TS, DEPI DOCSIS MPT, and DEPI DOCSIS PSP by an EQAM can consume memory resources.
Like reference numbers and designations in the various drawings indicate like elements.